1. Field of the Invention
This invention relates to a cam shaft for an internal combustion engine, the cam shaft comprising two shaft elements of which one is disposed inside the other and which are capable of limited angular and/or axial movement relative to one another, so as to change the relationship between first cam elements connected to one of the shaft elements and second cam elements connected to the other shaft element.
Such a cam shaft is used in an internal combustion engine, with the inlet valves of the engine operated by the cam elements connected to one of the shaft elements and the exhaust valves of the engine operated by the cam elements connected to one of the shaft elements and the exhaust valves of the engine operated by the cam elements connected to the other shaft element, in order to obtain a variation in the overlap of the opening periods of the inlet and exhaust valves. It is known that for an engine operating under part load and low speed conditions, only a small overlap between the opening periods of the inlet and exhaust valves is desirable, whilst at higher speeds, gas dynamics dictate that a greater overlap is required. If the inlet valves and exhaust valves of an engine are operated by separate cam shafts, a change in the angular relationship between the cam shafts can readily be achieved by relatively simple measures, for example by operating on a driving belt by means of an adjusting device, to increase or decrease the operative length of the belt between the two cam shafts. This expedient is not, however, applicable to a simple engine which does not have two separate cam shafts, and hence the provision for adjustment of the relationship of the cams to one another, to give favourable engine performance at both high and low speeds, has to be provided within a single cam shaft.
2. Description of Prior Art
A cam shaft including two shaft elements disposed one within the other is disclosed in EP 0254058 A2. In this, cam elements associated with the inner shaft element have cylindrical bores which slide on the outer surface of the outer shaft element, and in the region of their base circles have single radially extending fixing bolts which reach through peripheral slots in the outer shaft element to connect to the inner shaft element. This type of fixing can result in deformation of the inner shaft element in the region of the fixing bolts, when the cam is under torque loading. Damage can result, and loss of exact control of the cam timing.